In the United States, the Environmental Protection Agency (EPA) has promulgated Cluster Rules for cellulose pulp mills, and like mills. The Cluster Rules basically say that a cellulose pulp mill (e.g. kraft mill) has to collect streams of condensate resulting from the pulping operation containing at least 65% of the HAPs ("Hazardous Air Pollutants", defined by the EPA as MeOH (methanol), which is a surrogate for real HAPs which often includes more than 50 different kinds of components) and to treat these condensates so that 92% of this HAP (MeOH) is removed and destroyed by thermal oxidation (in a conventional thermal destruct unit). In order to comply with EPA Cluster Rules in the United States, cellulose pulp mills, including kraft pulp mills, have taken three basic approaches associated with condensate handling and heat recovery.
The first approach is to use hard piping. This segregates a specific quantity of process condensate containing HAPs and disposes of them into a defined and process specific aeration zone in a wastewater treatment plant of the mill. This approach is inexpensive, but does not typically allow optimum heat recovery.
The second approach is to recirculate process condensates. This concept attempts zero discharge, but is difficult to understand, and has not likely ever been successfully implemented.
The third general approach--to which the invention is directed--is to collect a specific quantity of process condensates, including containing HAPs, and to steam strip the condensates. One of the biggest problems associated with this general approach is what to do with the energy used, and coming out of the process. Preferably this energy can be integrated into existing process areas, such as evaporators or the like. However, this integration is very costly.
Another solution to the "what to do with the energy" problem is to use a pre-evaporator combined with a steam stripper using vapor compression technology, thus eliminating the need for energy integration. Another approach is to also make the stripper integration between two different steam pressures (e.g. 150-50 psig), thus enabling the recovery of practically all of the energy from one level to another. This can be a very efficient alternative, providing that the steam is not used in a back-pressure steam turbine. However, a significant drawback of both of these solutions is the system operating temperature (typically between about 260-320 degrees F.) compared to the temperature of the condensates being stripped (typically between about 200-220 degrees F.). This requires heating of the incoming foul condensate while cooling the outgoing stripped condensate, necessitating the use of very large multi-pass multiple indirect heat exchangers. The costs of these indirect heat exchangers can sometimes exceed the cost of the stripper column, and the indirect heat exchangers are sensitive to fouling of the heating surfaces thereof.
According to the present invention, the third general approach set forth above is utilized, but at a fraction of the cost of an indirect heat exchanger system, and in such a way that the system is totally insensitive to scale formation or pluggage due to impurities in the condensate.
According to one aspect of the present invention a method of treating foul condensate is provided comprising: (a) collecting foul condensate containing Hazardous Air Pollutants (HAPs) in a pulp mill; the foul condensate having a temperature of between about 140-180 degrees F.; (b) passing the foul condensate into direct contact with heated vapor at a plurality of series connected stations to gradually heat the foul condensate to a temperature desirable for steam stripping; (c) steam stripping the heated foul condensate to produce a high temperature clean condensate; (d) flashing the high temperature (e.g. about 250-350.degree. F., e.g. about 300.degree. F.) clean condensate in a plurality of flash stations to produce a heated vapor and a lowered temperature clean condensate; and (e) using the heated vapors from (d) to heat the foul condensate in each of the stations from (b).
Preferably (b) and (e) are practiced to provide the highest temperature vapor from (e) to the highest temperature foul condensate from (b), with progressively lower temperature vapors from (e) used for progressively lower temperature condensate stations from (b). Also, preferably (b) and (e) are each practiced at between two-ten stations, and preferably the same number of stations are utilized during the practice of each of (b), (d), and (e). Preferably high pressure steam (e.g. 130-200 psig) is provided for stripping, and the vapor from stripping is reboiled, producing lower pressure (e.g. 30-100 psig) steam. The temperature of the clean condensate discharged from (e) is preferably between about 190-210 degrees F.
Also, under normal circumstances (c) is practiced using steam at a pressure of about 130-200 psig (e.g. about 150 psig). The method may further comprise (f) producing a vapor during (c), and (g) using steam at a pressure of about 30-100 psig (e.g. about 50 psig) to reboil the vapor from (c). In the method (g) may be practiced to produce a discharge containing the majority of HAPs, and there may be the further procedure of (h) destroying the HAPs in the discharge (typically by thermal destruction in a conventional thermal unit).
According to another aspect of the invention a foul condensate treatment and heat recovery system is provided comprising: a plurality of series connected direct contact condensers including a first condenser connected to a pump and a source of foul condensate, and a last condenser; a steam stripper connected to the last condenser; a clean condensate discharge line extending from the steam stripper; a plurality of series connected flash tanks connected to the clean condensate discharge line, and including a first flash tank operatively closest to the steam stripper and receiving the highest temperature input clean condensate, and a last flash tank, each flash tank having a flashed vapor outlet; and the first flash tank vapor outlet operatively connected to the last condenser to heat the foul condensate therein, and the last flash tank vapor outlet connected to the first condenser to heat the foul condensate therein. Any intermediate flash tanks and condensers are connected to each other so that the respective vapor temperatures of the vapor outlets are matched with (connected to) comparable temperature condensers. The vapor outlet from the stripper is preferably connected to a reboiler.
The system typically includes at least one intermediate flash tank and condenser, and may further comprise a reboiler. The vapor outlet from the stripper may be connected to the reboiler. Also, the system may comprise a HAPs thermal destruct unit, and the boiler may comprise a discharge connected to the thermal destruct unit. A pump is typically provided between each of the condensers.
It is the primary object of the present invention to provide an efficient and effective method and system for complying with the Cluster Rules. This and other objects of the invention will become clear from a detailed inspection of the invention and from the appended claims.